Low temperature sintering of nickel ferrite powders

ABSTRACT

Method of producing sintered nickel ferrite powder having steps of mixing the particles of iron oxide and nickel oxide with an alkali metal borate mineralizer, compacting the mixture to produce green compact and heating the compact at temperatures less than about 1400° C. Resulting product which may be in the form of a non-consumable electrode for electrolysis of alumina, exhibits satisfactory mechanical properties and electrical properties with enhanced chemical stability while being produced at significantly lower sintering temperatures than previously employed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for sinteringnickel ferrite powder, more particularly to including a mineralizer inthe nickel ferrite powder which allows for reduced sinteringtemperatures to achieve high densification of the powder into a shapedcomponent.

2. Prior Art

Conventional production of aluminum by the Hall-Heroult process involveselectrolysis of alumina dissolved in molten salts of aluminum fluorideand sodium fluoride using carbon anodes. The anodes are suspended in abath of the electrolytic fluid. Electric current supplied to the anodesresults in production of electrons for reducing the alumina to aluminumwhich accumulates as a molten aluminum pad. The molten aluminum pad actsas a liquid metal cathode. During this process, the carbon anodescontinually react with oxygen released during the reduction of aluminato produce CO₂ thereby decomposing and consuming the anodes. In view ofthe consumption of carbon, there have been attempts to producenon-consumable material for the anodes that would be resistant tooxidation and attack by the molten salt bath. Replacement of carbonanodes with inert anodes should provide a highly productive cell designand reduce capital costs. Significant environmental benefits are alsopossible because inert anodes do not produce CO₂ or CF₄ emissions. Theuse of a dimensionally stable inert anode also allows for efficient celldesigns with a shorter anode-cathode distance and consequent energysavings.

The most significant challenge to the commercialization of inert anodetechnology is the anode material. The anode material must withstand theharsh environment of the Hall cell. In particular, the material shouldnot react with or dissolve to any significant extent in the electrolytebath. It must not react with oxygen or corrode in an oxygen-containingatmosphere and should be thermally stable at temperatures of about 1000°C. The anode material should have good mechanical strength and highelectrical conductivity at the smelting cell operating temperature,about 950-970° C., so that the voltage drop at the anode is low. Inaddition, aluminum produced with the inert anodes should not becontaminated with constituents of the anode material to any appreciableextent and the anode material should be relatively inexpensive.

Inert anodes formulated from nickel oxide (NiO) and iron oxide (Fe₂O₃)have found some success in electrolysis of alumina. These anodes aretypically manufactured by blending powders of the metal oxides,calcining the mixture followed by grinding to a fine particle size. Thefine particles are blended with organic binders and/or dispersants andformed into an anode shape. Once the component is formed in its “green”state, it is fired at temperatures typically at about 1350° C. or higherin air or reduced oxygen atmospheres. During the firing process, thebinder decomposes by oxidation or pyrolysis. At temperatures above 600°C., the particles of iron oxide and nickel oxide react and fuse togetherto form nickel ferrite. It has been found that sintering at temperatureswell above 1200° C., such as over about 1400° C., is needed to maximizedensification of the anodes.

High densification of the nickel ferrite anode is required to reduce thesurface area of the anode and thereby minimize the opportunity forcorrosion on the surface and within the anode. Full densification may beachieved at 1500° C., however significant energy input is required toachieve such high temperatures. In addition, the thermal energy which isstored in the anode following high temperature sintering creates thermalstresses within the anode that can result in failure of the anode. Ithas also been found that nickel ferrite anodes sintered at about 1500°C. to achieve full densification contain relatively large grains at thesurface of the anode. However, in the interior of the anode, the grainsizes are significantly reduced with concomitant higher quantities ofgrain boundaries. Grain boundaries have different chemistry from thebulk component and provide a route for the electrolyte bath and thealumina to enter the anode and effect corrosion. In addition, an anodesuspended in an electrolyte bath has a portion which is normally outsideof the bath and exposed to air. The interaction between air and thenickel ferrite anode also tends to create non-uniform grain sizes thatcompound the thermal stresses in the anode.

One objective of the present invention is to reduce the sinteringtemperature required to achieve full densification of nickel ferriteanodes. At lower sintering temperatures, the thermal stresses in thesintered anodes are lower and the anodes are more mechanically stable. Arelated need is to control the grain size in nickel ferrite anodes to bemore uniform throughout the anodes with minimal grain boundaries.

SUMMARY OF THE INVENTION

This need is met by the method of the present invention of producing asintered nickel ferrite component in which particles of iron oxide andnickel oxide are blended with an alkaline metal borate mineralizer. Themixture is shaped into a green compact. The green compact is sintered attemperatures less than about 1400° C. It has been found that use of analkaline metal borate mineralizer allows for sufficient densification attemperatures less than 1400° C. with larger grain sizes and greateruniformity through the thickness of the sintered component as comparedto components produced according to the prior art. The alkali metalborate may be sodium borate, lithium borate or cesium borate. In someinstances it may be helpful to use incipient wetting of the iron oxideparticles by mixing the alkaline metal borate in an aqueous solutionwith the particles. A binder may be mixed with the particles tostrengthen the green compact prior to sintering. The compact may includeabout 50 to 75 wt. % iron oxide and about 25 to 50 wt. % nickel oxidewith sufficient concentration of the mineralizer to include about 0.025to 1.6 wt. % boron. The present invention is particularly suited forproducing an inert anode for use in electrolysis of alumina.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d is a set of micrographs of a nickel ferrite componentproduced according to the prior art;

FIGS. 2 a-2 d is a set of micrographs of a nickel ferrite componentproduced according to the present invention using a lithium boratemineralizer;

FIGS. 3 a-3 d is a set of micrographs of a nickel ferrite componentproduced according to the present invention using a sodium boratemineralizer; and

FIG. 4 is a graph showing the change in normalized size of the nickelferrite components versus sintering temperature of FIGS. 1-3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for producing a non-consumableelectrode suitable for use in the production of metals by electrolyticreduction of their oxides in a molten salt bath. The method provides ahigh densification of the electrode to achieve chemical inertness andgood electrical conductivity with acceptable mechanical properties.Inert electrodes produced according to the present invention aresuitable for producing metals such as aluminum, lead, magnesium, zinc,zirconium, titanium, lithium, thalium, silicon and the like, generallyby electrolytic reduction of an oxide or other salt of the metal. Whenreferring to any numerical range of values, such ranges are understoodto include each and every number and/or fraction between the statedrange minimum and maximum.

In the present invention, powders of NiO and Fe₂O₃ are blended in amixer. The relative composition of iron oxide and nickel oxide in themixture may vary such as about 50 to 75 wt. % iron oxide and 25 to 50wt. % nickel oxide. The blended powders may be ground to a smallerparticle size before being transferred to a furnace where they arecalcined, typically for about 12 hours at about 1250° C. The calcinationproduces a mixture having nickel ferrite spinel and NiO phases. Thecalcined mixture is blended with a mineralizer of an alkali metalborate. The mixture of mineralizer and metal oxides is ground in a ballmill or the like to an average particle size of approximately 10microns.

The fine particles may be blended with a polymeric binder, dispersantsand water to make a slurry in a spray dryer. In an alternativeembodiment, the alkali metal borate is added to the metal oxide powdermixture along with the binder instead of prior to the grinding step. Thepolymeric binder added to the mixture is preferably an organic materialsuch as polyvinyl alcohol, acrylic acid polymers, glycol such aspolyethylene glycol and a polyvinyl acetate, polyisobutylenes,polycarbonates, polystyrenes, polyacrylates and mixtures and copolymersthereof. The dispersants may also be organic materials. When used, about0.1-10 parts by weight of the organics (binder and dispersant) are addedto 100 parts by weight of the metal oxides. Preferably, about 3-6 partsby weight of the organics are added to 100 parts by weight of the metaloxides. The slurry typically contains about 60 wt. % solids and about 40wt. % water. Spray drying the slurry produces dry agglomerates that aretransferred to a V-blender for final mixing.

The V-blended mixture is isostatically pressed, for example at 20,000psi, into anode shapes. The pressed shapes are sintered in air in afurnace heated to a temperature of less than about 1400° C. for about2-4 hours. The sintering temperature may be less than about 1200° C. Thesintering furnace preferably contains an air atmosphere, but reducedoxygen or inert (e.g. argon) atmospheres may also be used. The sinteringprocess burns out the polymeric binder from the anode shapes andachieves sufficient densification of the anode.

The alkali metal borate mineralizer is believed to facilitate transferof material from high energy to low energy sites. Crystal growth duringsintering is affected by particle characteristics, temperature,atmosphere, type of mineralizer and amount of mineralizer present.Alkaline metal borate compounds have been found to be suitablemineralizers for use in the present invention. In particular, boratecompounds of sodium, lithium and cesium are suited for use in thepresent invention. Anhydrous alkaline metal borates are preferred overhydrous borates having a water constituent that increases the volume ofmaterial which is lost during densification. Very low amounts ofmineralizer have been found to be effective in achieving low temperaturesintering of nickel ferrite components. For example, the amount of boronfrom the mineralizer may be about 0.025-1.6 parts by weight boron,preferably 0.045-0.3 parts by weight boron, per 100 parts by weight ofthe metal oxides. Higher boron levels may be used without a deleteriouseffect. At such low amounts of mineralizer, it may be helpful to useincipient wetting to mix the mineralizer with the metal oxides or metaloxide/binder mixture. As such, the alkaline metal borate is presented inan aqueous solution of 0.5-0.7 wt. % boron for mixing with the metaloxides.

While nickel and iron oxides are preferred compounds for producing aninert anode, other suitable compounds may be oxides of tin, zinc,lithium, zirconium, chromium or tantalum. Other suitable compounds ofthe metals include metal salts that are converted to oxides when exposedto oxygen and elevated temperatures include halides, carbonates,nitrates, sulfates and acetates.

Inert electrodes made in accordance with our invention are preferablyinert anodes useful in electrolytic cells for metal production operatedat temperatures in the range of about 750-1080° C. A particularlypreferred cell operates at a temperature of about 900-980° C.,preferably about 950-970° C. An electric current is passed between theinert anode and a cathode through a molten salt bath comprised of anelectrolyte and an oxide of the metal to be collected. In a preferredcell for aluminum production, the electrolyte comprises aluminumfluoride and sodium fluoride and the metal oxide is alumina. The weightratio of sodium fluoride to aluminum fluoride is about 0.7 to 1.25,preferably about 1.0 to 1.20. The electrolyte may also contain calciumfluoride and/or lithium fluoride.

Although the invention has been described generally above, theparticular examples give additional illustration of the product andprocess steps typical of the present invention.

EXAMPLES Example 1 Comparative

A mixture containing 68.2 wt. % Fe₂O₃, 29.8 wt. % NiO and 2 wt. %mixture of polyethylene glycol (PEG) and polyvinyl alcohol (PVA) binderswas pressed into a compact and sintered in air at 1500° C. for twohours. FIGS. 1A and 1B are photomicrographs of the resultant componentafter polishing near its edge and in the bulk of the component,respectively. FIGS. 1C and 1D are photomicrographs of the componentafter thermal etching near the edge and in the bulk, respectively. Itcan be seen that away from the edge area, the grain sizes aresignificantly reduced with increased quantity of grain boundaries.

Example 2

A mixture of 68.2 wt. % Fe₂O₃, 29.8 wt. % NiO, 2 wt. % mixture of PEGand PVA binders and 0.045 wt. % boron from lithium borate were pressedinto a die to produce a green compact. The compact was sintered in airat 1200° C. for two hours. FIGS. 2A and 2B are photomicrographs of theresultant component as polished in the edge area and in the bulk of thecomponent, respectively. FIGS. 2C and 2D are photomicrographs of thecomponent after thermal etching near the edge and in the bulk,respectively. It can be seen that the quantity of grain boundaries doesnot increase within the bulk of the component from the edge area andthat the grain sizes are larger and more uniform than those produced inExample 1.

Example 3

Example 2 was repeated except that sodium borate was used in place oflithium borate. FIGS. 3A and 3B are photomicrographs of the resultantcomponent as polished in the edge area and in the bulk of the component,respectively. FIGS. 3C and 3D are photomicrographs of the componentafter thermal etching near the edge and in the bulk, respectively.Again, uniformly sized large grains are present through the thickness ofthe component as compared to the component of Example 1.

In each of Examples 1-3, the change in size of the product wasdetermined by dilatometry during the sintering process. A graph of thenormalized change in dimensions versus sintering temperature isreproduced in FIG. 4. It will be appreciated that at temperatures over900° C., greater densification occurred with the components producedaccording to the present invention using sodium borate and lithiumborate as opposed to the control. At 1200° C., the components of thepresent invention were nearly fully densified, while the controlcomponent was only about 60% densified. In addition, the electrodesproduced according to examples 2 and 3 are believed to exhibitmechanical properties and electrical properties which were comparable tothat of the control with improved chemical stability.

1. A method of producing a sintered nickel ferrite component comprisingthe steps of: (a) mixing particles of iron oxide and nickel oxide withan alkali metal borate; (b) compacting the mixture produced in step (a)to provide a green compact; and (c) heating the green compact at atemperature less than about 1400° C. to produce a sintered component. 2.The method of claim 1, wherein the alkali metal borate is selected fromthe group consisting of sodium borate, lithium borate and cesium borate.3. The method of claim 1, wherein the alkali metal borate of step (a)comprises an aqueous solution of the alkali metal borate such that step(a) comprises incipient wetting of the particles.
 4. The method of claim1, wherein the green compact has a shape suitable for an anode of analuminum smelting bath.
 5. The method of claim 1, wherein step (a)further comprises mixing a binder with the particles.
 6. The method ofclaim 5, wherein the alkali metal borate is mixed with the particlesprior to adding the binder.
 7. The method of claim 5, wherein the alkalimetal borate and the binder are simultaneously mixed together with theparticles.
 8. The method of claim 1, wherein the compact comprises about50-75 wt. % iron oxide and 25-50 wt. % nickel oxide.
 9. The method ofclaim 1, wherein a sufficient concentration of the alkali metal borateis added so that the compact comprises about 0.025-1.6 wt. % boron. 10.The method of claim 1, wherein a sufficient concentration of the alkalimetal borate is added so that the compact comprises about 0.045-0.3 wt.% boron.
 11. A sintered nickel ferrite component produced according tothe method of claim
 1. 12. An inert anode for use in a molten salt bathcomprising a sintered composition comprising nickel ferrite and analkali metal borate.
 13. The anode of claim 12, wherein the alkali metalborate is selected from the group consisting of sodium borate, lithiumborate and cesium borate.
 14. The anode of claim 12, wherein theconcentration of boron in the sintered composition is about 0.025-1.6wt. %.
 15. The anode of claim 12, wherein the concentration of boron inthe sintered composition is about 0.045-0.3 wt. %.
 16. An electrolyticcell for producing aluminum comprising: a molten salt bath comprising anelectrolyte and alumina; an anode comprising the inert anode of claim12; and a cathode.
 17. The electrolytic cell of claim 16, wherein saidmolten salt bath comprises aluminum fluoride and sodium fluoride.
 18. Amethod of producing metal by passing a current between an anode and acathode through a molten salt bath comprising an electrolyte and anoxide of the metal to be produced, said anode comprising the inert anodeof claim
 12. 19. The method of claim 18, wherein the metal is aluminum.20. The method of claim 19, wherein the molten salt bath comprisesaluminum fluoride and sodium fluoride.